This invention relates to testing for and treating defects in vision, and particularly to treating amblyopia.
Amblyopia is a defect in visual acuity of an eye that persists after refractive error in the eye has been corrected and any pathological obstacle to vision has removed. It is a leading cause of impaired visual acuity in one eye, affecting about 2% of persons in the U.S. Unless successfully treated, amblyopia constitutes a potential handicap.
A variety of approaches to treating amblyopia have been tried. Approaches based promoting the use of the amblyopic eye include, for example, optical penalization, administration of cycloplegic drugs, CAM visual stimulation and occlusion of the better eye.
Generally, in optical penalization, lenses are used to blur the visual acuity of the better eye and to aid the visual acuity of the amblyopic eye. Cycloplegic drugs (such as atropine) are administered to blur the vision of the better eye by preventing accommodation and decreasing the depth of focus. Such drugs must be used with caution, however; in particular, the defocused image resulting from the use of atropine is sufficient to produce stimulus deprivation amblyopia, and the atropine administration can result in light sensitivity and dissociation.
In one known approach, a near-vision fixation distance is found where the visual axes cross in convergent squint, and the patient's perception of this physiological diplopia is used to encourage the use of binocular vision. This brings the amblyopic eye into use with the other eye at one fixation distance, and results in an improvement in acuity. This method requires a high degree of interest and co-operation in the patient.
In an occlusion therapy known as Red filter occlusion, the amblyopic eye is occluded for most of each day except for a short period, increasing to half a day, during which the better eye is occluded and a dark red filter (Wratten filter No. 92) is placed before the amblyopic eye. With the red filter in place over the amblyopic eye and the occluder in place over the better eye, the patient is encouraged to use the amblyopic eye as much as possible. Evidence from clinical evaluations of this method is very conflicting, and the underlying theory behind it is doubtful.
In another general approach, a central after-image is created in the dominant eye, and transferred to the amblyopic eye. The patient is then asked to try to locate the after-image at the point of fixation, and to see smaller fixation letters. The procedure is repeated when the after-image fades, and the acuity is measured after several repeats. It appears that the best results are obtained when the starting acuity is 6/24 or better, and in those cases when the binocular vision and acuity has deteriorated following previous improvement achieved by other orthoptic procedures.
In one known approach, a special ophthalmoscope (termed "euthyscope") is used to center a wide ring after-image on the fovea of the amblyopic eye. The fovea itself is spared the after-image, while the surrounding retina, including the eccentrically fixing area, is desensitized by the after-image. The patient is then asked to look at individual letters in decreasing sizes with true foveal fixation. This method requires a substantial amount of practitioner's time, and is tiring for the patient, and to be effective, it must be rigorously applied. For these reasons this method has never gained wide acceptance.
In another approach, central fixation of the amblyopic eye is encouraged by asking the patient to see Haidinger's brushes. Initially, an empty bright blue field of rotating polarized light is used, and then a fixation letter or other target is introduced.
Occlusion of the non-amblyopic eye, `direct occlusion`, is a long established method and has proved to produce good results in many cases. The usual method is total occlusion, in which an attempt is made to ensure that no light enters the eye and the amblyopic eye is brought into use. Schedules for occlusion vary widely among practitioners, but generally, where total direct occlusion is thought to be the best procedure, the occlusion is maintained throughout the day, or at least during waking hours, for several months for a maximum response. Checks of the acuity of both eyes generally are carried out weekly throughout the treatment period, and according to one general rule of thumb no more than 1 week of constant patching for every year of the patient's age should be permitted between examinations. Although occlusion is sometimes necessary, it is not in all cases the preferred method. Where good binocular vision is the ultimate aim, covering one eye may not be the best way to start, since occlusion promotes monocular use.
Occlusion of the dominant eye is not entirely successful as a cure for amblyopia, not least because it is difficult to marshall full cooperation by patients and their families. The patient, typically a child or young adult, is asked to participate in daily activities, including school, with greatly reduced visual function. Patients often are embarrassed by their appearance while wearing the patch and are teased by their peers, and some patients suffer from skin irritation or allergic reactions to contact with the patch material. Moreover, there is in occlusion treatments a sense of being greatly removed from the source of therapy; the patients and their parents lose touch with the therapist and are not given the frequently necessary reinforcement and encouragement. Any of these factors can lead to distress and a decision not to continue with treatment, or to noncooperation by the patient.
Some of the problems encountered in total occlusion therapy can be alleviated by reducing the duration of each day's period of occlusion. Minimal occlusion therapy requires the child to wear a totally opaque patch for only 20 to 30 minutes a day, during which time the child plays some kind of visually demanding game. It is important with this form of therapy that the child concentrate hard on the task and that the task be as fine and difficult as the child is able to undertake.
An apparatus known as the "CAM visual stimulator" was devised to treat amblyopia by using intense visual stimulation of the amblyopic eye for short periods of time. In this therapeutic approach, occlusion is used during treatments but not at other times. The apparatus consists of a base on which is fitted one of six circular high contrast gratings, each of a different spatial frequency. The grating has a transparent cover on which the patient is encouraged to draw or to play drawing games to ensure that he concentrates on the grating. The patient draws on the transparent cover while the better eye is occluded and the grating beneath the cover is slowly rotated.